1. Field of the Invention
The present invention relates to a radar dome (radome) structure which provides a high level of ballistic protection for an electronics assembly and antenna elements mounted therein, together with high transparency to electromagnetic signals propagating therethrough.
2. Description of the Prior Art
It is desirable to mount a radar antenna and associated electronic units (transmitting, receiving, signal processing circuitry, etc.) inside a radome structure to protect these elements from weather and ballistic projectiles without adversely affecting the electrical performance of the radar system.
Radomes for ballistic protection of land and ship based radar antennas have conventionally utilized fiberglass and/or aramid materials in single layer or composite wall structures. The composite or sandwich wall construction is desirable for increasing the structural rigidity of the radome so that it can withstand high wind loading, typically in excess of 100 km/hr, without excessive deflection. A typical configuration is described in U.S. Pat. No. 4,783,666, entitled "PROTECTIVE SHIELD FOR AN ANTENNA ARRAY", issued Nov. 8, 1988, to H. Ast et al. The materials used must be as transparent as possible at the radio frequencies (RF) utilized by the radar to minimize the energy loss of the RF signals propagating through the radome.
In complex radar antennas this loss is very critical to the operation of the system and must be minimized. The allowable RF energy loss through the radome wall limits the maximum thickness of the radome which in turn establishes the maximum ballistic protection. To achieve the highest possible level of ballistic protection, the radome material must have a high ballistic limit, low RF signal loss, and ideally will also have low weight density. Fiberglass and aramid materials provide acceptable levels of ballistic protection and have low weight density, but the RF signal loss is relatively high which, in many cases, degrades the performance of the antenna below acceptable limits.
A high ballistic strength material has recently been developed and marketed under the trademark SPECTRA.RTM. by Allied Fibers, a division of Allied Signal, Inc. The SPECTRA.RTM. material is formed of extended chain polyethylene (ECPE) fibers, and provides higher ballistic protection and lower signal loss than conventional fiberglass and aramid materials. Examples of laminates formed from ECPE fibers are disclosed in U.S. Pat. No. 4,820,568, entitled "COMPOSITE AND ARTICLE USING SHORT LENGTH FIBERS", issued Apr. 11, 1989, to G. Harpell et al, and assigned to Allied Signal, Inc.
A composite radome wall structure utilizing ECPE laminates is capable of providing sufficient ballistic protection for many applications while maintaining the two-way RF insertion loss over a range of operating frequencies and scan angles below a low value on the order of 0.13 dB. However, where ballistic protection against heavy projectiles such as .50 caliber bullets is required, increasing the total thickness of the ECPE laminate in the radome wall structure to a sufficient value to stop the projectiles increases the RF insertion loss to an unacceptably high level.